Membrane-type MMPs enable extracellular matrix permissiveness and mesenchymal cell proliferation during embryogenesis[unreadable] [unreadable] MT1-MMP-deficient mice temporarily survive despite a severe skeletal phenotype caused by cellular collagen indigestion. This led us to question if collagen remodeling ultimately is required for development and growth or if compensatory mechanisms are enabled to facilitate matrix remodeling. We established by in situ hybridization that the molecular relative of MT1-MMP, MT3-MMP, is expressed abundantly in skeletal tissues in a pattern overlapping, in part, that of MT1-MMP. To test the hypothesis that MT3-MMP can ameliorate the loss of collagenolytic activity, we generated mice with targeted disruption of MT3-MMP. These mice display a modest, yet significant phenotype in adulthood, with diminished growth of the cranium and limbs when compared to wildtype littermates. Mice deficient for both MT3-MMP and MT1-MMP, on the other hand, display profound developmental defects in the appendicular and craniofacial skeleton, including cleft palate, and die immediately after birth. The observed phenotypic traits indicate that the loss of a second collagenolytic enzyme is catastrophic, and demonstrate that MT3-MMP is the molecule responsible for survival of MT1-MMP deficient mice. Consistent with these findings, MT3-MMP confers collagenolytic activity to collagenase-deficient MT1-MMP-null cells when expressed in these cells in vitro. Taken together, the data identify MT3-MMP as a major mesenchymal collagenase in vivo, and demonstrate that collagen remodeling is essential for embryonic development and post-natal life.[unreadable] [unreadable] MT1-MMP is required for efficient tumor dissemination in experimental metastatic disease.[unreadable] [unreadable] Aside from the essential role in embryonic and postnatal matrix remodeling of skeletal and peri-skeletal tissues MT1-MMP is also found in pathophysiological conditions. Accordingly, membrane-type I matrix metalloproteinase (MT1-MMP) is associated with multiple forms of cancer including mammary cancer which has a high frequency of metastasis to the skeleton and as such constitutes a serious challenge to skeletal health. To directly evaluate the significance of MT1-MMP expression in tumor progression and metastasis using a genetically induced cancer model, we crossed MT1-MMP-deficient mice to MMTV-polyoma virus middle T-antigen (PyMT) mice. Expression of PyMT in the MT1-MMP-deficient background consistently resulted in hyperplasia of the mammary gland as seen in wild-type PyMT littermates. Because of the reduced life span of MT1-MMP deficient mice, we applied orthotopic transplantation of PyMT+ glands into the cleared mammary fat pad of syngeneic recipient mice to evaluate tumor progression and metastasis in MT1-MMP-deficient tumors. Mutant tumors were palpable earlier than wild-type tumors and grew to the experimental end point size quicker than control tumors, but demonstrated markedly reduced ability to metastasize to the lungs of recipient mice. Accordingly, MT1-MMP-deficient mice displayed an overall reduction in metastasis count of 50%. Importantly, MT1-MMP was expressed solely in the stroma of PyMT-induced tumors and those metastatic nodules that formed in the lungs were devoid of MT1-MMP expression. In an effort to identify the underlying cause for the reduction in metastasis we analyzed the collagenolytic activity stromal cells. Stromal fibroblasts isolated from MT1-MMP-deficient tumors did not degrade type I collagen suggesting that efficient dissemination of tumor cells is dependent on stromal cell remodeling of the tumor environment. The data demonstrate directly that MT1-MMP-mediated proteolysis by stromal cells is important in the metastatic process and contributes to the dissemination of MT1-MMP negative cancer cells. Equally important, these findings also suggest that anti-MMP therapeutic strategies to inhibit tumor formation are unlikely to succeed given the prolific growth of tumors derived from MT1-MMP deficient mice.